Big definitive post coming up here...
-At current quality levels (256kb/s), you get an average of about 120songs/GB.
-iPod Classic holds 160GB but contains mechanical parts. Lots of storage but more sensitive to vibration/handling/temperature.
-Newest iPod Nano comes in 8/16GB and has an internal speaker and a video camera as well as the ability to tune into FM radio.
You sound like you're most interested in the iPod Touch. Here are the rest of the facts.
-1st generation - available in 8/16/32GB, essentially an iPhone with no phone, BlueTooth nor camera. Ships with version 1 OS, some later models shipped with additional apps (Mail, etc). Will charge with FireWire/USB.
-2nd generation - available in 8/16/32GB, increases raw processor speed 33%, adds external volume controls and an internal speaker, supports Nike+ and audio-in with proper accessories (handy for Skype). Ships with version 2 OS. Updating this model to version 3 OS also enables BlueTooth functionality. Will charge with USB only.
-3rd generation - available ONLY in 32/64GB, all of the above plus Voice Control and Voiceover (accessibility features) and controllable from headset remote. Completely redesigned (presumably even faster) processor and graphics just like the iPhone 3Gs (which would mean 2x the RAM, too). Ships with version 3 OS. Will charge with USB only.
Other important facts:
-All iPods will play MP3s anywhere from 8-320kb/s. And while iPods will not play WMA files, the Windows version of iTunes will convert your non-DRM'd WMAs into AAC, which then can be loaded onto an iPod.
-Reformatting an iPod will remove any illicit software from that iPod, but it will not repair any damage already done by driving components (like WiFi) past their 'normal' limits, etc.
-The iPhone 3G and 3Gs use "aGPS" (WiFi and cell tower triangulation) to get reasonably close to your current location, but they then use a dedicated GPS chip to pinpoint your exact location after the initial guesstimate.
More later, if needed (or PM for in-depth).
--Patrick